1. Field of the Invention
The present invention relates to a system for sequentially lighting fluorescent lamps. This system can be preferably employed in a color printer, a color scanner or the like.
2. Description of the Prior Art
In an optical reader such as a color printer, a color scanner or the like, tricolor light sources are applied to an original document to be read, sequentially first in red, second in green and third in blue. In this event, the original document is normally moved relative to the light sources for the scanning thereof so that reflected light from the light sources may be read out by an image pickup means such as a CCD (charge coupled device) or the like. A plurality of fluorescent lamps each for independently emitting light in one of the aforementioned three colors are generally employed as the light sources.
FIG. 1 illustrates an electric circuit of one of the conventional lighting systems.
Red, green and blue fluorescent lamps 1, 2 and 3 respectively are connected at their respective filaments R0, G0 and B0 commonly to the first winding 4a of a power transformer 4. The other filaments R1, G1 and B1 of the fluorescent lamps 1, 2 and 3 are independently connected to the second winding 4b, to the third one 4c and to the fourth one 4d of the transformer 4, respectively. The primary winding 4p of the transformer 4 is connected to a driving circuit 5 through a line l1 so as to be electrically biased through the driving circuit 5.
The line l1 is also connected to three starting circuits 9, 10 and 11 through contacts S1 S2 and S3 of switching circuits 6, 7 and 8, respectively so as to be electrically biased by the starting circuits 9, 10 and 11. The switching circuits 6, 7 and 8 sequentially connect and disconnect the line l1 to the starting circuits 9, 10 and 11 under control of the driving circuit 5 through a line l2 every predetermined time. The switching circuits 6, 7 and 8 each comprise, for example, relays and the like.
In this circuit, a power switch S0 is initially turned on. When the driving circuit 5 has been electrically biased, the transformer 4 is charged with electric current therethrough. In this event, filament voltage V0 common to all the fluorescent lamps 1 to 3 is applied to the filaments R0, G0 and B0 thereof, whereas filament voltage V1, V2 and V3 are independently applied to the other filaments R1, G1 and B1 of the fluorescent lamps 1 to 3, respectively. As a result, the filaments of the red, green and blue fluorescent lamps 1, 2 and 3 respectively are heated and continue to be heated in use.
Upon lapse of a period required for preheating the filaments, the driving circuit 5 drives the switching circuits 6, 7 and 8 so that the first, second and third contacts S1, S2 and S3 may be sequentially and cyclically connected or disconnected. As a result, the first, second and third starting circuits 9, 10 and 11 are sequentially connected to the line l1 so that the first, second and third starting voltage V4, V5 and V6 are sequentially independently applied between both the filaments R0-R1 of the red fluorescent lamp 1, between those G0-G1 of the green fluorescent lamp 2 and between those B0-B1 of the blue fluorescent lamp 3 through current-limiting capacitors C1, C2 and C3, respectively. Consequently, the corresponding fluorescent lamps 1 to 3 are selectively turned on and off so that the scanning and readout of the original document may be conducted with the use of three different colors.
In the above described conventional system, however, it is necessary to independently provide three switching circuits 6, 7 and 8 for sequentially lighting the corresponding fluorescent lamps 1, 2 and 3. Since these appliances inevitably occupy considerable space in an apparatus employing therein the conventional system, such an apparatus has been manufactured undesirably at high cost. There has been, therefore, an increased demand for the system for lighting the fluorescent lamps which enables the apparatus to be of a small size at reduced production cost.